The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the automatic determination of scan field of view, display field of view, and patient centering parameters prior to a scan.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Prior to a scan the operator typically enters a number of scan parameters that affect the location and quality of the reconstructed image. These include such geometric scan parameters as the scan field of view ("SFOV"), the display field of view ("DFOV") and patient centering parameters X.sub.off and Y.sub.off.
The corrections made during reconstruction (often called the beam hardening coefficients) are dependent on the SFOV for the scan. Large, medium, and small SFOVs have different correction coefficients that are optimized for patients up to 48 cm, 35 cm, and 25 cm respectively. The DFOV is the region within the SFOV that is to be reconstructed. Generally the DFOV should be as small as possible and should be centered over the anatomy of interest. This results in the largest image for the anatomy of interest. Because a CT system normally projects maximum X-ray signal at the center of the SFOV, it is desirable to carefully center the patient within this region to obtain optimum image quality. Any residual mis-centering requires offsets in X and Y to position the smallest possible DFOV for the anatomy of interest within the SFOV.
Quite often the operator does not adjust these geometric parameters prior to each scan to optimize performance. This may occur because the operator does not have the necessary information upon which to base such settings, or because the time is simply not taken to determine the proper settings and make them. In any event, less than optimal images are often the result.